The present invention relates to a power steering system having a bypass valve which directs excess fluid flow from a pump to reservoir.
A known power steering system includes a pump, a steering control valve which is actuated by turning of a vehicle steering wheel, and a power steering motor for assisting in moving the vehicle wheels in response to actuation of the control valve. The pump is driven by the engine and provides fluid at a flow rate which varies as a function of the speed of operation of the engine. At high engine operating speeds, the output of the pump is substantially greater than at low engine operating speeds. A bypass valve directs excess fluid flow from the pump to the reservoir.
In a power steering system disclosed in U.S. Pat. No. 4,691,797 issued Sept. 8, 1987 and entitled Fluid Flow Control Apparatus for a Power Steering System, hydraulic fluid from a power steering pump flows through a flow control orifice to a power steering control valve. The flow control orifice creates a hydraulic pressure drop proportional to the rate of fluid flow. A flow control valve bypasses fluid in response to the hydraulic pressure drop.
In the power steering system disclosed in U.S. Pat. No. 4,691,797, the bypass flow control valve is controlled (i) in response to the fluid flow rate through the flow control orifice and (ii) in response to a pressure drop created by a small pilot fluid flow from the outlet of the pump through a variable orifice. The variable orifice is provided by a solenoid actuated valve located in the pilot fluid flow conduit. The solenoid actuated valve is controlled in response to vehicle speed, and acts to reduce fluid flow to the power steering motor as vehicle speed increases and while steering is occurring. Since the solenoid valve is actuated as a function of vehicle speed rather than demand for fluid by a power steering motor, the flow of fluid to the power steering motor is not reduced during operation of the vehicle at low speed in nonsteering conditions. This results in the power steering pump requiring an unnecessarily large amount of power at low vehicle operating speeds in nonsteering conditions.
The use of a solenoid actuated valve in a power steering system, in the manner disclosed in the aforementioned U.S. Pat. No. 4,691,797, could be objectionable. This is because under at least certain operating conditions, a solenoid actuated valve may not be as durable as a mechanical or fluid pressure actuated valve. In addition, it is believed that a mechanical or fluid actuated valve is simpler and easier to manufacture and install in a power steering system than a solenoid actuated valve.